1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine which includes a noble metal chip bonded either on a tip of a center electrode or a ground electrode.
2. Description of Related Art
To improve durability and performance of an spark plug for an internal combustion engine, a spark plug using a platinum (Pt) alloy as either a center or ground electrode has been proposed and is in use. Recently, there has been a tendency make both center and ground electrodes smaller in diameter and elongated in order to further improve sparking performance and ignitability in consideration of cleaner exhaust and lean combustion. When the Pt alloy electrode is used, for example, in a form of a thin and elongated center electrode, the spark gap tends to be enlarged and spark malfunction often occurs because of dissipation of the electrode.
As a counter measure to this problem, it has been proposed to bond a noble metal chip on either the center or ground electrode. The noble metal chip may be bonded on the electrode by resistance welding. However, when the noble metal chip is bonded on the electrode by resistance welding, the welded portion may be damaged due to thermal stress caused by a difference in thermal expansion coefficients of the noble metal and the electrode.
The noble metal chip may be bonded by laser welding. In laser welding, a laser beam having a high energy density is focused on a junction of the noble metal chip and the electrode. Both of the noble metal and a metallic material of the electrode are melted by the high density laser beam and make a molten bond at the junction. However, a ratio of the noble metal melted into the electrode material in the molten bond is heavily dependent on the energy of the laser beam, and accordingly durability of a spark plug becomes variable depending on the laser beam energy. For example, if the noble metal chip is made of iridium (Ir) and the electrode to which the noble metal chip is bonded is made of nickel (Ni), a ratio of Ir to Ni in the molten bond is very small because the melting point of Ir is much higher than that of Ni (Ir: 2450.degree. C.; Ni: 1450.degree. C.). When the Ir ratio in the molten bond is very small, thermal stress at the junction is not alleviated. If the laser energy is increased to melt Ir in a higher ratio, Ni evaporates and makes voids in the molten bond and a large depression is formed on the periphery of the molten bond, because the melting point of Ir and the boiling point of Ni are not far apart (the boiling point of Ni: 2700.degree. C.).